1. Technical Field
The present disclosure is directed to systems and methods for separating one or more components from a gaseous mixture and, more particularly, to systems and methods for separating carbon dioxide from a gaseous system, e.g., atmospheric air, using hydrophobic porous/nonporous hollow fibers that are in contact with an absorbent solution. The present disclosure further relates to systems and methods for regenerating the absorbent solution on a periodic basis.
2. Background Art
Gas separation using facilitated transport membranes (FTMs) has been the subject of considerable research for many years. The types of FTMs investigated generally fall into the following three categories: (1) immobilized liquid membrane (ILM), (2) solvent-swollen polymer membrane, and (3) fixed-carrier membranes. Major advantages of FTMs over conventional polymeric membranes include enhanced selectivity and permeability for the target species because of reversible reactions between the carriers in FTM and the target species. This characteristic makes FTM especially attractive when the target species in the feed gas mixture exists in low concentrations because, to accomplish the separation and/or purification task, the limited transmembrane driving force is generally too small for conventional polymeric membranes.
Though generally considered to be the least stable FLM configuration, ILM has been widely investigated for facilitated transport of carbon dioxide using various carriers. Ward and Robb made a pioneering study on CO2 permeation through a thin layer of carbonate/bicarbonate aqueous solution. Otto and Quinn and later Suchdeo and Schultz made theoretical analyses of CO2 transport through carbonate/bicarbonate ILMs. [Each of the noted research efforts is identified in the list of References appended hereto.] Other investigators used amines as the carriers and/or ion-exchange membranes as the substrates. LeBlanc et al. and later Way et al. studied facilitated transport of CO2 in ion-exchange membranes using various organic amine counterions. Teramoto et al. used monoethanolamine (MEA) solutions, while Guha et al. and Davis and Sandall used diethanolamine (DEA) solutions immobilized in porous substrates as ILMs to study CO2 transport. Matsuyama et al. studied CO2 transport through a plasma-polymerized ion-exchange substrate employing ethylenediamine (EDA) as the carrier.
Despite the attractive features of and extensive studies on ILMs for gas separation, commercial gas separation applications have been limited. Major work is still needed to improve the membrane permeances and demonstrate significantly longer operating life. In ILMs, the liquid solution is physically trapped in, but not chemically bonded to, the support matrix. The low stability can be a result of liquid washout and/or the evaporation of the liquids into the gas phases during operation. Various strategies have been employed to alleviate the problems of carrier loss and ILM drying out. Hughes et al. tried to circumvent the stability problem of an Ag+-containing ILM for olefin-paraffin separation by periodically regenerating it. A more common practice when aqueous solutions are used as the ILM fluid is to humidify both the feed and sweep gas streams simultaneously. Another alternative is to use low volatility and hygroscopic liquids, such as poly(ethylene glycol) (PEG) or glycerol as the major component in the ILM fluid.
Chen et al. have reported that hydrophilic poly(vinylidene fluoride) (PVDF)-based Na2CO3-glycerol ILMs are stable when challenged with feed streams of very low relative humidities (RH). Because of the relatively low carrier concentrations and high viscosity of the glycerol-based ILM fluid, the Na2CO3-glycerol ILM showed high CO2/N2 selectivities, but relatively low CO2 permeances. The glycerol-based ILM could be useful for CO2 removal from gas streams containing low concentrations of CO2 if its CO2 permeance were to be significantly increased.
Additional ILM-based work has been reported by Chen et al., in which efforts were aimed at developing a membrane for CO2 separation from breathing mixtures for space-walk applications. Conventionally, this separation is done by adsorption/reaction using adsorbents/reagents discarded when saturated. Glycine-Na-glycerol and glycine-Na-carbonate-glycerol ILMs were investigated for CO2 separation for spacesuit applications. As an amino acid salt, glycine-Na is environmentally friendly. Like other amines, glycinate ion forms labile complexes with carbon dioxide, but not with oxygen or nitrogen.
Spacesuit applications impose significant limitations on selection of ILM liquid and carrier species. Because the feed gas is normally not completely humidified (i.e., RH<100%), the ILM must be stable when the feed stream RH is relatively low. Also, to conserve oxygen, the membrane should have very high CO2/O2 selectivity (e.g., >2000) at low RHs. Moreover, the ILM components should be completely environmentally benign. Therefore, the most studied amines in the literature (e.g., MEA, DEA, and EDA) are not suitable for such application because of their relatively high volatilities and irritative nature.
Glycine has been used as an additive in carbonate/bicarbonate solutions for the selective removal of CO2 from industrial gas streams. LeBlanc et al. demonstrated that glycine salt can be a better carrier species for CO2 than carbonate in ion-exchange substrate-based ILMs. In addition, glycine salts have been incorporated into polymeric membranes for enhanced CO2 separation from gas streams containing CO2 and H2. Ho disclosed a CO2 separating polymeric membrane fabricated from poly(vinyl alcohol) and glycine salts (e.g., glycine-K and glycine-Li).
Kowali and Sirkar have explored the use of glycerol carbonate with added carriers as a liquid membrane for CO2 separation. Glycerol with added carriers in the liquid membrane mode was also explored for CO2 selective separation by Chen et al. In unpublished experiments, Kowali studied CO2 absorption and desorption in one single module as a function of time. In addition, Chen et al. have studied the use of glycerol and/or glycerol carbonate with added carrier (sodium glycinate) as an absorbent solution, instead of using such materials in the liquid membrane mode.
Despite efforts to date, a need remains for systems and methods for effective gas separation and, in particular, for separation of carbon dioxide from atmospheric air. In addition, separation systems and methods are needed which may be effectively operated over extended periods, and which may be regenerated in an efficient manner.